One of the benefits I've drawn from Less Wrong so far - via posts like The Simple Truth - is more solid foundations for my beliefs. Since I study physics, I wasn't particularly worried about philosophical arguments against the scientific method anymore - science seemed to work, after all - but of those doubts that remained, many more still got (apparently) dispelled or dissolved.

That said, I never had doubts that fundamental. Could anybody really live that way? I don't have a coherent mental model for such a situation. Take Mad-Eye Moody in HPMoR with his constant paranoia, as in "Unless, of course, that's what they want you to think." - could such a character really function in the real world? Could they eat food without starving themselves to death out of paranoia? If you are paranoid about both X -> Y and X -> ~Y, how do you decide whether to do Y or ~Y? (It seems more plausible to me that Moody doesn't actually exhibit doubts that fundamental, than that he or anyone else could function properly despite those doubts.)